Vehicle navigation system and route guidance method

ABSTRACT

A vehicle navigation system includes a road-between-guidance-target-points configuration determining unit ( 9 ) for determining a configuration of a road between two successive guidance target points whose information is extracted from information stored in a guidance target point storing unit ( 8 ). A continuous guidance threshold distance setting unit ( 10 ) sets a threshold distance for continuous guidance based on the road configuration by using a table or the like. A distance-between-guidance-target-points calculating unit ( 11 ) calculates a distance between the two successive guidance target points. When the calculated distance between the two successive guidance target points is shorter than the set threshold distance for continuous guidance, a route guidance unit ( 12 ) provides continuous guidance for users through a guidance unit ( 16 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a navigation system for vehicleand a route guidance method that enable a driver who is driving avehicle on a road to safely and smoothly pass through guidance targetpoints such as intersections.

[0003] 2. Description of Related Art

[0004] A prior art navigation system for vehicle has a function ofcontinuously providing guidance for a driver who is driving a vehicle ona road. When the vehicle has to continuously make a right-hand turn orleft-hand turn two or more times while traveling within a region locatedat a predetermined distance from the current position of the vehicle, byusing the continuously guidance function, the prior art navigationsystem provides guidance on a first guidance target point andsimultaneous guidance on a second guidance target point, such as “Make aright-hand turn 100 m ahead and then make a left-hand turn”, before thevehicle reaches the first guidance target point. In this case, the priorart navigation system determines whether or not a distance between twosuccessive guidance target points is equal to or less than a thresholddistance and, when the distance is equal to or less than the thresholddistance, provides such continuous guidance.

[0005] The above-mentioned threshold distance can be arbitrarily set.However, when the threshold distance is set to a long one, the prior artnavigation system may often provide continuous guidance needlessly andtherefore redundant guidance can cause the driver to have momentarylapses of attention for the first guidance target intersection at whichthe driver has to make a turn. In contrast, when the threshold distanceis set to a short one, essentially needed continuous guidance cannot beprovided and the driver may often have an inconvenience of being forcedto make a sudden lane change or right or left hand turn after thevehicle has passed through the first guidance target intersection.

[0006] Thus, the setting of the threshold distance for continuousguidance is important and it is therefore desirable that the setthreshold distance complies with the road on which the vehicle istraveling. In accordance with a prior art method of providing continuousguidance in consideration of the suitability of the threshold distancefor the road on which the vehicle is traveling, the threshold distancefor continuous guidance is properly preset according to a differencebetween “general road” and “highway”, each of which can be an attributeof the road between the two successive guidance target intersections(for example, refer to Japanese patent application publication (TOKKAI)No. 2000-39330).

[0007] Thus, the prior art continuous guidance method has the step ofusing the threshold distance for continuous guidance that is properlyset according to a difference between “general road” and “highway”.However, in accordance with this prior art continuous guidance method,the threshold distance for continuous guidance is kept at constant forthe road on which the vehicle is traveling, and is not set so that itsuits with conditions of the road. For example, a problem with the priorart continuous guidance method is that when the vehicle is traveling ona general road, the setting of the threshold distance cannot beimplemented in consideration of the configurations of variousintersections on the general road. A further problem is that even whenthe vehicle is traveling on a highway, the setting of the thresholddistance cannot be implemented in consideration of a configuration ofthe highway.

SUMMARY OF THE INVENTION

[0008] The present invention is proposed to solve the above-mentionedproblems, and it is therefore an object of the present invention toprovide a vehicle navigation system for and a route guidance method of,when there exist continuous guidance target points on a searched-forroute, providing appropriate and continuous guidance according to aconfiguration or conditions of a road on which a vehicle is traveling,thereby making it possible for drivers to pass though intersectionssafely and smoothly.

[0009] In accordance with the present invention, there is provided avehicle navigation system provided with a guidance target storing unitfor storing information about guidance target points, which is extractedfrom information about a preset route extending from a current positionof a vehicle to a destination and which is based on map information, anda guidance unit for providing guidance on the route by voice or withdisplay, the system including: a road-between-guidance-target-pointsconfiguration determining unit for determining a configuration of a roadbetween two successive guidance target points whose information isextracted from the information stored in the guidance target storingunit; a continuous guidance threshold distance setting unit for settinga threshold distance for continuous guidance based on the roadconfiguration determined by the road-between-guidance-target-pointsconfiguration determining unit; adistance-between-guidance-target-points calculation unit for calculatinga distance between the two successive guidance target points whoseinformation is extracted from the information stored in the guidancetarget storing unit; and a route guidance unit for providing continuousguidance through the guidance unit when the calculated distance betweenthe two successive guidance target points is shorter than the thresholddistance for continuous guidance set by the continuous guidancethreshold distance setting unit.

[0010] As a result, when two or more guidance target points successivelyexist on a road on which the vehicle is traveling, the vehiclenavigation system can provide appropriate and continuous guidance forthe driver according to a configuration of the road on which the vehicleis traveling. Therefore the vehicle navigation system can prevent thedriver from making sudden lane changes and can also prevent needlesscontinuous guidance, so that the driver can pass through intersectionssafely and smoothly.

[0011] Further objects and advantages of the present invention will beapparent from the following description of the preferred embodiments ofthe invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram showing the functional configuration ofa vehicle navigation system in accordance with embodiment 1 of thepresent invention;

[0013]FIG. 2 is a block diagram showing the hardware configuration ofthe vehicle navigation system in accordance with embodiment 1 of thepresent invention;

[0014]FIG. 3 is a flow chart showing continuous guidance processingperformed by the vehicle navigation system in accordance with embodiment1 of the present invention;

[0015]FIG. 4 is an example of a table showing a relationship between thenumber of lanes and a threshold distance L for continuous guidance, inorder to explain the flow chart of FIG. 3;

[0016]FIG. 5 is an explanatory diagram showing an advantage offered bycontinuous guidance provided by the vehicle navigation system inaccordance with embodiment 1 of the present invention;

[0017]FIG. 6 is another explanatory diagram showing the advantageoffered by continuous guidance provided by the vehicle navigation systemin accordance with embodiment 1 of the present invention; and

[0018]FIG. 7 is a block diagram showing the functional configuration ofa vehicle navigation system in accordance with embodiment 2 of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The invention will now be described with reference to theaccompanying drawings.

Embodiment 1

[0020]FIG. 1 is a block diagram showing the functional configuration ofa vehicle navigation system in accordance with embodiment 1 of thepresent invention. In the figure, a map information storing unit 2, amap information acquiring unit 3, a current position detecting unit 4, aroute searching unit 5, a route storing unit 6, a guidance target pointextracting unit 7, a guidance target point storing unit 8,road-between-guidance-target-points configuration determining unit 9, acontinuous guidance threshold distance setting unit 10,distance-between-guidance-target-points calculating unit 11, a routeguidance unit 12, a guidance unit 16 provided with voice messagegenerating unit 13, a voice output unit 14, and a display unit 15, andan operation unit 17 are connected to a control unit 1.

[0021] The control unit 1 controls various calculations and the whole ofthe navigation system. The map information storing unit 2 storesdigitalized map data, such as link data and node data. The mapinformation acquiring unit 3 acquires map information stored in the mapinformation storing unit 2. The current position detecting unit 4detects the current position of a moving object (e.g., a vehicle) inwhich the navigation system is mounted. The route searching unit 5 setsa route between two points, i.e., between the vehicle's current pointindicating the current position of the vehicle and defined by map datastored in the map information storing unit 2 and a destination point.The route storing unit 6 then stores the route set by the routesearching unit 5 therein.

[0022] The guidance target point extracting unit 7 extracts informationabout guidance target points, such as guidance target intersections, foreach of which guidance needs to be provided, for the route whoseinformation is stored in the route storing unit 6. Hereafter, thepreferred embodiments of the present invention will be explained bytaking guidance target intersections as an example of guidance targetpoints. The guidance target point storing unit 8 then stores theinformation about the guidance target intersections whose information isextracted by the guidance target point extracting unit 7. Theroad-between-guidance-target-points configuration determining unit 9determines a configuration of each of roads associated with the guidancetarget intersections on each of which determination of whether or notcontinuous guidance is to be provided is performed. The continuousguidance threshold distance setting unit 10 sets a threshold distancefor continuous guidance on each road according to the road configurationdetermined by the road-between-guidance-target-points configurationdetermining unit 9.

[0023] The distance-between-guidance-target-points calculating unit 11calculates a distance between any two successive ones of all theguidance target intersections on each of which determination of whetheror not continuous guidance is to be provided is performed. The routeguidance unit 12 performs control operations, such as determination ofwhether continuous guidance or single guidance is to be provided anddetermination of the direction in which the vehicle should travel whenthe vehicle reaches each of the guidance target intersections, on eachof all the guidance target intersections stored in the guidance targetpoint storing unit 8. The voice message generating unit 13 has voiceinformation storing unit for storing words, phrases, and so on requiredfor generating voice guidance messages, as voice waveform data, and,when the navigation system provides voice guidance, selects voicewaveform data about words, phrases, and so on indicating a guidancemessage corresponding to the voice guidance, and combines them so as togenerate a voice guidance message.

[0024] The voice output unit 14 informs the user of the guidance messagegenerated by the voice message generating unit 13 by voice. The displayunit 15 displays both a map stored in the map information storing unit 2and the route stored in the route storing unit 6. When the vehicleapproaches one of all the guidance target intersections stored in theguidance target point storing unit 8, the display unit 15 enlarges anddisplays the map including a region that is located in the vicinity ofthe guidance target intersection. The operation unit 17 has switchesthrough which the user (i.e., the driver) is allowed to operate thevehicle navigation system when the user presets the destination, forexample. The operation unit 17 receives and manages input signalsgenerated by those switches.

[0025]FIG. 2 is a block diagram showing the hardware configuration ofthe navigation system, which corresponds to the functional configurationof FIG. 1. As shown in FIG. 2, a map information storing unit 22, a GPS(Global Positioning System) receiver 23, a bearing sensor 24, a distancesensor 25, a voice output unit 26, a display 27, and an input device 28are connected to a control unit 21.

[0026] The control unit 21 corresponds to the control unit 1 of FIG. 1,and performs control of the whole of the navigation system and variouscalculations. The map information storing unit 22 corresponds to the mapinformation storing unit 2 of FIG. 1, and is provided with a DVD-ROM(digital versatile disc-read only memory) for storing digitalized mapdata and a reading unit for reading data from the DVD-ROM. The GPSreceiver 23 receives electric waves from satellites so as to detect thecurrent position of the moving object (e.g., a vehicle) in which thenavigation system is mounted. The bearing sensor 24 is a sensor thatdetects the direction (i.e., the heading) in which the moving object istraveling. The distance sensor 25 is a sensor that detects the distancetraveled by the moving object. The combination of the GPS receiver 23,the bearing sensor 24, and the distance sensor 25 corresponds to thecurrent position detecting unit 4 of FIG. 1.

[0027] The voice output unit 26 corresponds to the voice output unit 14of FIG. 1, and outputs a guidance message or the like by voice. Thedisplay 27 corresponds to the display unit 15 of FIG. 1, and is providedwith a liquid crystal display or the like, for example. The display 27displays map information, the route, guidance messages, and so on. Theinput device 28 corresponds to the operation unit 17 of FIG. 1, andgenerates and outputs signals when the user operates this input device28 so as to cause the navigation system to perform desired operations.

[0028] The control unit 21 is provided with a CPU (central processingunit) 31, a ROM (read only memory) 32, a RAM (random access memory) 33,a display control unit 34, and an I/O unit (input/output unit) 35. TheCPU 31 performs calculations for route searching, extracting ofinformation about guidance target intersections and so on. The ROM 32stores program constants and so on which the CPU 31 uses when performingoperations. In other words, the ROM 32 stores a program associated withcontinuous guidance, which will be described later, in addition to otheroriginal programs used for performing navigation operations, such assearching for a route from the current position of the vehicle to thedestination and guidance that is provided for the user while the vehicleis traveling along the route searched for.

[0029] While the CPU 31 performs operations, a program, map data, and soon are expanded into the RAM 33 or calculation results are written intothe RAM 33. The display control unit 34 controls the display 27. The I/Ounit 35 interfaces between the control unit 21 and the external variousunits 22 to 28. The map information acquiring unit 3, the routesearching unit 5, the route storing unit 6, the guidance target pointextracting unit 7, the guidance target point storing unit 8, theroad-between-guidance-target-points configuration determining unit 9,the continuous guidance threshold distance setting unit 10, thedistance-between-guidance-target-points calculating unit 11, the routeguidance unit 12, the voice message generating unit 13, etc. as shown inFIG. 2 are implemented via processes performed by the control unit 21.

[0030] Next, the basic operation of the whole of the navigation systemwill be explained with reference to FIG. 1. Before starting driving thevehicle, the user can operate the switches of the operation unit 17 soas to input and set desired data, such as a destination, to thenavigation system. The current position detecting unit 4 detects thecurrent position of the vehicle. On the other hand, map data are storedin the map information storing unit 2, and the map information acquiringunit 3 acquires necessary map information from the map informationstoring unit 2. The route searching unit 5 searches for a route from thedetected current position of the vehicle to the destination input andset through the operation unit based on those map data, and sets theroute to the vehicle navigation system. The set route between the twopoints, i.e., between the current position and the destination is storedin the route storing unit 6. The guidance target point extracting unit 7extracts information about guidance target intersections for whichguidance needs to be provided from information about the route stored inthe route storing unit 6. The extracted information about the guidancetarget intersections is then stored in the guidance target point storingunit 8.

[0031] The route guidance unit 12 determines whether or not continuousguidance is to be provided for each of all the guidance targetintersections whose information is stored in the guidance target pointstoring unit 8. The determination performed by the route guidance unit12 will be described later. The display unit 15 displays a map stored inthe map information storing unit 2, the route between the two pointsstored in the route storing unit 6, an enlarged map of a region locatedin the vicinity of a guidance target point whose information is storedin the guidance target point storing unit 8 when the vehicle approachesthe guidance target point, and so on, or displays them for guidance.Furthermore, when the vehicle approaches a guidance target point, thevoice output unit 14 outputs a corresponding voice guidance messagegenerated by the voice message generating unit 13 and provides the voiceguidance message for the driver. The control of the above-mentionedoperations is carried out by the control unit 1.

[0032] Next, the determination process of determining whether or notcontinuous guidance is to be provided for each of the guidance targetintersections will be explained with reference to FIG. 3. FIG. 3 is aflow chart showing an example of the determination process ofdetermining whether or not continuous guidance is to be provided foreach of the guidance target intersections. The process shown in FIG. 3includes the step of determining whether or not the route guidance unit12 needs to provide continuous guidance for each of the guidance targetintersections whose information is stored in the guidance target pointstoring unit 8, and the step of providing guidance on each of theguidance target intersections when determining that the route guidanceunit 12 needs to provide continuous guidance for each of the guidancetarget intersections.

[0033] The road-between-guidance-target-points configuration determiningunit 9, in step ST1, acquires the following information I about both afirst guidance target intersection A that is located just ahead of thevehicle, and a second guidance target intersection B that is locatedahead of the vehicle and is next to the first guidance targetintersection A, the information about the two successive guidance targetintersections A and B being stored in the guidance target point storingunit 8.

[0034] (Information I): the number of lanes of a link between the twosuccessive guidance target intersections A and B.

[0035] The reason why the acquired information I is the number of lanesis because when the link between the two successive guidance targetintersections A and B has many lanes, there is a possibility that thedriver makes a lane change many times and it is necessary to set thethreshold distance L for continuous guidance to a long one in thefollowing steps.

[0036] The continuous guidance threshold distance setting unit 10 then,in step ST2, determines the threshold distance L for continuous guidancebased on the information about the number of lanes of the link betweenthe two successive guidance target intersections A and B acquired instep ST1, by using a table, as illustrated in FIG. 4, showing arelationship between the number of lanes of the link between the twosuccessive guidance target intersections and the threshold distance Lfor continuous guidance.

[0037] The distance-between-guidance-target-points calculating unit 11,in step ST3, calculates the distance between the two successive guidancetarget intersections A and B, and the route guidance unit 12 comparesthis calculated distance with the threshold distances L for continuousguidance determined in step ST2. When the comparison result shows thatthe distance between the two successive guidance target intersections Aand B is equal to or less than the threshold distance L for continuousguidance (i.e., when YES in step ST3), the route guidance unit 12advances to step ST4 in which the route guidance unit 12 providescontinuous guidance (e.g., a voice message or the like), such as “Make aright hand turn 100 m ahead and then make a left hand turn at the nextintersection”, through the guidance unit 16, for example.

[0038] In contrast, when the comparison result shows that the distancebetween the two successive guidance target intersections A and B isgreater than the threshold distance L for continuous guidance (i.e.,when NO in step ST3), the route guidance unit 12 advances to step ST5 inwhich the route guidance unit 12 provides single guidance, such as “Makea right hand turn 100 m ahead” through the guidance unit 16, forexample.

[0039] As previously mentioned, the information I acquired in step ST1is the number of lanes of the link between the two successive guidancetarget intersections A and B. As an alternative, theroad-between-guidance-target-points configuration determining unit 9 canacquire the following information instead of the number of lanes of thelink, and can determine the threshold distance L for continuous guidancebased on the information, in step ST2.

[0040] (1) The width of the road between the two successive guidancetarget intersections A and B.

[0041] The reason why the acquired information is the road width (i.e.,the width of the road) is because when the road width is large, there isa high possibility that the driver makes a lane change many times and itis therefore preferable that the threshold distance L for continuousguidance is set to a longer one.

[0042] (2) The difference between the direction in which the vehiclewill be headed at the first guidance target intersection A and that inwhich the vehicle will be headed at the second guidance targetintersection B.

[0043] The reason why the acquired information is the difference betweenthe headings at the guidance target intersections A and B is becausewhen the difference between the headings at the guidance targetintersections A and B is equal to or larger than a predetermined value(for example, when the headings are opposite to each other, such as whenthe vehicle will make a right hand turn at one of the guidance targetintersections A and B and will make a left hand turn at the otherguidance target intersection), there is a high possibility that thedriver makes a lane change and it is therefore preferable that thethreshold distance L for continuous guidance is set to a longer one.

[0044] (3) The angle (referred to as the turn angle) with which thevehicle will make a turn at the first guidance target intersection A.

[0045] The reason why the acquired information is the turn angle isbecause when the turn angle at the first guidance target intersection Ais equal to or less than a predetermined value, there is a possibilitythat the vehicle will be traveling in a substantially straight linealong a highway, for example, and will pass through the first guidancetarget intersection and arrive at the second guidance targetintersection in a short time, and it is therefore preferable that thethreshold distance L for continuous guidance is set to a longer one.

[0046] In step ST2, instead of using the table, the continuous guidancethreshold distance setting unit 10 can set the threshold distance L forcontinuous guidance by using either of the following methods:

[0047] (1) When the number of lanes, the road width or the differencebetween the headings at the guidance target intersection A and B, whichis acquired in step ST1, is equal to or greater than a predeterminedvalue or when the turn angle at the first guidance target intersection,which is acquired in step ST1, is equal to or less than a predeterminedvalue, the continuous guidance threshold distance setting unit 10 canset the threshold distance L for continuous guidance to longer than areference distance. In contrast, when the number of lanes, the roadwidth or the difference between the headings at the guidance targetintersection A and B is less than the predetermined value or when theturn angle at the first guidance target intersection is greater than thepredetermined value, the continuous guidance threshold distance settingunit 10 can set the threshold distance L for continuous guidance toshorter than the reference distance. The continuous guidance thresholddistance setting unit 10 then prestores the set threshold distance L forcontinuous guidance therein. The reference distance is set in advance,and the continuous guidance threshold distance setting unit 10 cancorrect the reference distance based on the information acquired in stepST1, as previously mentioned.

[0048] (2) The continuous guidance threshold distance setting unit 10prestores a relational expression that associates either of the numberof lanes, the road width, the difference between the headings at theguidance target intersections A and B, or the turn angle at the firstguidance target intersection with the threshold distance L forcontinuous guidance, and calculates and sets the threshold distance Lfor continuous guidance corresponding to the actual information acquiredin step ST1 according to the above-mentioned relational expression. Forexample, in the case of the number of lanes, the continuous guidancethreshold distance setting unit 10 has such a relational expression as“threshold distance L for continuous guidance=the number of lanes*100m”. As a result, when the number of lanes of the road is 1, thecontinuous guidance threshold distance setting unit 10 sets thethreshold distance L for continuous guidance to 100 m, and when thenumber of lanes of the road is 4, the continuous guidance thresholddistance setting unit 10 sets the threshold distance L for continuousguidance to 400 m. The continuous guidance threshold distance settingunit 10 can be provided with a similar relational expression for each ofthe road width, the difference between the headings at the guidancetarget intersections A and B, and the turn angle at the first guidancetarget intersection.

[0049] The above explanation of embodiment 1 provided by referring toFIG. 3 is given by taking the navigation system (i.e., a product) as anexample. Furthermore, in accordance with embodiment 1, a route guidancemethod which this navigation system employs can include the processes asshown in FIG. 3. In other words, step ST1 is the process of determininga configuration of the road between the two successive guidance targetintersections A and B whose information is extracted from the guidancetarget point storing unit 8, and step ST2 is the process of setting thethreshold distance L for continuous guidance based on the roadconfiguration determined in step ST1. Furthermore, step ST3 is theprocess of calculating the distance between the two successive guidancetarget intersections A and B whose information is extracted from theguidance target point storing unit 8, and steps ST3 and ST4 are theprocess of providing continuous guidance for the user through theguidance unit 16 when the calculated distance between the two successiveguidance target intersections A and B is equal to or shorter than theset threshold distance L for continuous guidance. Steps ST3 and ST5 arethe process of providing single guidance for the user through theguidance unit 16 when the calculated distance between the two successiveguidance target intersections A and B is longer than the set thresholddistance L for continuous guidance.

[0050] For example, when the road between the two successive guidancetarget intersections A and B has three lanes, as shown in FIG. 5, suchsingle guidance at the first guidance target intersection A as “Make aleft hand turn” can cause the driver to drive the vehicle along a pathdesignated by a dotted line, and can force the driver to make a lanechange immediately before the vehicle reaches the second guidance targetintersection B that is closer to the destination than the first guidancetarget intersection A. In contrast, in accordance with embodiment 1,when the road between the two successive guidance target intersections Aand B has a larger number of lanes as shown in FIG. 5, even if thedistance S between the two successive guidance target intersections Aand B is longer than a conventionally-set threshold distance forcontinuous guidance, the route guidance unit 12 can provide continuousguidance, such as “Make a left hand turn and then make a right hand turnat the next intersection”, so that the driver can determine that he orshe should drive the vehicle along a lane designated by a solid line inadvance. In addition, even when the vehicle is traveling on a wide roadand the driver is forced to make a lane change or even when thedifference between the direction in which the vehicle will be headed atthe first guidance target intersection and that in which the vehiclewill be headed at the second guidance target intersection is large, theroute guidance unit 12 can provide continuous guidance, so that thedriver can determine the direction in which the vehicle should be headedin advance.

[0051] When the turn angle θ at the first guidance target intersection Ais small, as shown in FIG. 6, because there is a possibility that thevehicle passes through the first guidance target intersection A withoutslowing down and reaches the second guidance target intersection B in ashort time, such single guidance at the first guidance targetintersection A as “Make a right hand turn” can leave the driver littletime for making a right hand turn at the second guidance targetintersection B. In contrast, in accordance with embodiment 1, when theturn angle θ at the first guidance target intersection A is small, asshown in FIG. 5, even if the distance S between the two successiveguidance target intersections A and B is longer than aconventionally-set threshold distance for continuous guidance, the routeguidance unit 12 can provide continuous guidance, such as “Make a righthand turn and then make a right hand turn at the next intersection”, sothat the driver can drive the vehicle to pass through the secondguidance target intersection B while keeping calm.

[0052] As mentioned above, in accordance with this embodiment 1, thevehicle navigation system determines a configuration of a road betweentwo successive guidance target intersections on each of whichdetermination of whether or not continuous guidance is to be provided isperformed according to either of the number of lanes of the road, theroad width, the difference between the headings at the two successiveguidance target intersections, or the turn angle at the first one of thetwo successive guidance target intersections, sets a threshold distanceL for continuous guidance according to the determined road configurationby using a conversion table or by using a method of correcting apredetermined reference distance, or the like, calculates the distancebetween the two successive guidance target intersections, and providescontinuous guidance for the driver when the calculated distance betweenthe two successive guidance target intersections is shorter than thethreshold distance L for continuous guidance. When two or more guidancetarget intersections successively exist on the road on which the vehicleis traveling, the vehicle navigation system can provide appropriate andcontinuous guidance for the driver according to the configuration of theroad. Therefore, the vehicle navigation system can prevent the driverfrom making sudden lane changes and can also prevent needless continuousguidance, so that the driver can pass through intersections safely andsmoothly.

Embodiment 2

[0053]FIG. 7 is a block diagram showing the functional configuration ofa vehicle navigation system in accordance with embodiment 2 of thepresent invention. The same components as shown in FIG. 1 are designatedby the same reference numerals.

[0054] The vehicle navigation system in accordance with this embodiment2 differs from that in accordance with above-mentioned embodiment 1 inthat while the vehicle navigation system in accordance with embodiment 1sets a threshold distance L for continuous guidance based on informationabout a configuration of a road between two successive guidance targetintersections, such as the number of lanes of the road, the road width,the difference between the headings at the two successive guidancetarget intersections or the turn angle at the first one of the twosuccessive guidance target intersections, the vehicle navigation systemin accordance with this embodiment 2 sets the threshold distance L forcontinuous guidance based on a travel time required for the vehicle totravel between the two successive guidance target intersections.

[0055] To this end, the vehicle navigation system in accordance withthis embodiment 2 is provided withtravel-time-between-guidance-target-points determining unit 41 fordetermining a travel time required for the vehicle to travel between twosuccessive guidance target intersections, as shown in FIG. 7, instead ofthe road-between-guidance-target-points configuration determining unit 9of FIG. 1 (showing the functional configuration of the vehiclenavigation system in accordance with embodiment 1). The other componentsof the vehicle navigation system in accordance with this embodiment 2are the same as those of the vehicle navigation system in accordancewith embodiment 1, and are designated by the same reference numerals asshown in FIG. 1. Therefore the explanation of those components will beomitted hereafter.

[0056] Furthermore, the vehicle navigation system in accordance withthis embodiment 2 has the same hardware configuration as shown in FIG.2, and the travel-time-between-guidance-target-points determining unit41 can be implemented via processes performed by a control unit 21 asshown in FIG. 2.

[0057] In addition, the vehicle navigation system in accordance withthis embodiment 2 basically carries out the same continuous guidanceprocessing as shown in FIG. 3 and performed by that in accordance withembodiment 1. However, processes of steps ST1 and ST2 are different fromthose performed by the vehicle navigation system in accordance withabove-mentioned embodiment 1.

[0058] In other words, in the vehicle navigation system in accordancewith this embodiment 2, the travel-time-between-guidance-target-pointsdetermining unit 41, in step ST1, acquires either a travel time requiredfor the vehicle to travel on a road between first and second guidancetarget intersections A and B or a speed limit on the road, which isprestored in link information included in map data. The reason whyeither the travel time required for the vehicle to travel on the roadbetween the first and second guidance target intersections A and B orthe speed limit on the road is acquired in step ST1 is because when thetravel time is short or when the speed limit on the road between the twosuccessive guidance target intersections is high, there is a highpossibility that the vehicle passes through the first guidance targetintersection at a high speed and then arrives at the second guidancetarget intersection in a short time and it is therefore preferable toset the threshold distance L for continuous guidance to a longer one.

[0059] A continuous guidance threshold distance setting unit 10 then, instep ST2, determines the threshold distance L for continuous guidance byusing a table showing a relationship between either the travel timerequired for the vehicle to travel on the road between first and secondguidance target intersections A and B or the speed limit on the road,which is acquired in step ST1, and the threshold distance L forcontinuous guidance.

[0060] After that, the vehicle navigation system in accordance with thisembodiment 2 carries out steps ST3 and ST4 or ST5 in the same way asthat of above-mentioned embodiment 1 so as to provide continuousguidance or single guidance for the user. Instep ST2, instead of usingthe table, the continuous guidance threshold distance setting unit 10can set the threshold distance L for continuous guidance by using eitherof the following methods:

[0061] (1) When the above-mentioned travel time, which is acquired instep ST1, is equal to or less than a predetermined value or when theabove-mentioned speed limit, which is acquired in step ST1, is equal toor greater than a predetermined value, the continuous guidance thresholddistance setting unit 10 can set the threshold distance L for continuousguidance to longer than a reference distance. In contrast, when thetravel time is greater than the predetermined value or when the speedlimit is less than the predetermined value, the continuous guidancethreshold distance setting unit 10 can set the threshold distance L forcontinuous guidance to shorter than the reference distance. Thecontinuous guidance threshold distance setting unit 10 then prestoresthe set threshold distance L for continuous guidance therein.

[0062] (2) The continuous guidance threshold distance setting unit 10prestores a relational expression that associates the above-mentionedtravel time or speed limit with the threshold distance L for continuousguidance, and calculates and sets the threshold distance L forcontinuous guidance corresponding to the actual information acquired instep ST1 according to the above-mentioned relational expression.

[0063] The above explanation of embodiment 1 provided by referring toFIG. 3 is given by taking the navigation system (i.e., a product) as anexample. Furthermore, in accordance with embodiment 1, a route guidancemethod which this navigation system employs can include the processes asshown in FIG. 3. In other words, step ST1 is the process of determiningeither the travel time required for the vehicle to travel on the roadbetween the two successive guidance target intersections A and B whoseinformation is extracted from the guidance target point storing unit 8or the speed limit on the road, and step ST2 is the process of settingthe threshold distance L for continuous guidance based on the traveltime or speed limit determined in step ST1. Furthermore, step ST3 is theprocess of calculating the distance between the two successive guidancetarget intersections A and B whose information is extracted from theguidance target point storing unit 8, and steps ST3 and ST4 are theprocess of providing continuous guidance for the user through theguidance unit 16 when the calculated distance between the two successiveguidance target intersections A and B is equal to or shorter than theset threshold distance L for continuous guidance. Steps ST3 and ST5 arethe process of providing single guidance for the user through theguidance unit 16 when the calculated distance between the two successiveguidance target intersections A and B is longer than the set thresholddistance L for continuous guidance.

[0064] As can be seen from the above description, even when either thetravel time required for the vehicle to travel on a road between twosuccessive guidance target intersections A and B or the speed limit onthe road is small enough to raise a possibility that the vehicle reachesthe second guidance target intersection in a short time after passingthrough the first guidance target intersection, the driver can drive thevehicle to pass through the second guidance target intersection B whilekeeping calm.

[0065] As mentioned above, in accordance with this embodiment 2, thevehicle navigation system determines either the travel time required forthe vehicle to travel on a road between two successive guidance targetintersections or the speed limit on the road based on a travel time orspeed limit stored in each link information included in map informationdata, sets a threshold distance L for continuous guidance according tothe determined travel time or speed limit by using a conversion table orby using a method of correcting a predetermined reference distance, orthe like, calculates the distance between the two successive guidancetarget intersections, and provides continuous guidance for the driverwhen the calculated distance between the two successive guidance targetintersections is shorter than the threshold distance L for continuousguidance. When two or more guidance target intersections successivelyexist on the road on which the vehicle is traveling, the vehiclenavigation system can provide appropriate and continuous guidance forthe driver according to either the travel time required for the vehicleto travel on the road or the speed limit on the road. Therefore, thevehicle navigation system can prevent the driver from making sudden lanechanges and can also prevent needless continuous guidance, so that thedriver can pass through intersections safely and smoothly.

[0066] Many widely different embodiments of the present invention may beconstructed without departing from the spirit and scope of the presentinvention. It should be understood that the present invention is notlimited to the specific embodiments described in the specification,except as defined in the appended claims.

What is claimed is:
 1. A vehicle navigation system provided withguidance target storing means for storing information about guidancetarget points, which is extracted from information about a preset routeextending from a current position of a vehicle to a destination andwhich is based on map information, and guidance means for providingguidance on said route by voice or with display, said system comprising:road-between-guidance-target-points configuration determining means fordetermining a configuration of a road between two successive guidancetarget points whose information is extracted from the information storedin said guidance target storing means; continuous guidance thresholddistance setting means for setting a threshold distance for continuousguidance based on the road configuration determined by saidroad-between-guidance-target-points configuration determining means;distance-between-guidance-target-points calculation means forcalculating a distance between the two successive guidance target pointswhose information is extracted from the information stored in saidguidance target storing means; and route guidance means for providingcontinuous guidance through said guidance means when the calculateddistance between the two successive guidance target points is shorterthan the threshold distance for continuous guidance set by saidcontinuous guidance threshold distance setting means.
 2. The vehiclenavigation system according to claim 1, wherein saidroad-between-guidance-target-points configuration determining meansdetermines the configuration of the road between the two successiveguidance target points according to either one of following conditions:a number of lanes of the road, a width of the road, a heading differencebetween directions in which the vehicle will be headed at the twosuccessive guidance target points, respectively, and a turn angleshowing what degree the vehicle will make a turn at a first one of thetwo successive guidance target points, and said continuous guidancethreshold distance setting means sets the threshold distance forcontinuous guidance based on a table showing a relationship betweeneither one of the number of lanes, the road width, the headingdifference and the turn angle and the threshold distance for continuousguidance.
 3. The vehicle navigation system according to claim 1, whereinsaid road-between-guidance-target-points configuration determining meansdetermines the configuration of the road between the two successiveguidance target points according to either one of following conditions:a number of lanes of the road, a width of the road, a heading differencebetween directions in which the vehicle will be headed at the twosuccessive guidance target points, respectively, and a turn angleshowing what degree the vehicle will make a turn at a first one of thetwo successive guidance target points, and, when saidroad-between-guidance-target-points configuration determining meansdetermines that either one of the number of lanes of the road, the roadwidth and the heading difference is equal to or greater than apredetermined value or the turn angle is equal to or less than apredetermined value, said continuous guidance threshold distance settingmeans sets the threshold distance for continuous guidance to be longerthan a reference distance, and said continuous guidance thresholddistance setting means sets the threshold distance for continuousguidance to be shorter than a reference distance otherwise.
 4. Thevehicle navigation system according to claim 1, wherein saidroad-between-guidance-target-points configuration determining meansdetermines the configuration of the road between the two successiveguidance target points according to either one of following conditions:a number of lanes of the road, a width of the road, a heading differencebetween directions in which the vehicle will be headed at the twosuccessive guidance target points, respectively, and a turn angleshowing what degree the vehicle will make a turn at a first one of thetwo successive guidance target points, and said continuous guidancethreshold distance setting means sets the threshold distance forcontinuous guidance according to a relational expression showing arelationship between either one of the number of lanes of the road, theroad width, the heading difference and the turn angle and the thresholddistance for continuous guidance.
 5. A vehicle navigation systemprovided with guidance target point storing means for storinginformation about guidance target points, which is extracted frominformation about a preset route extending from a current position of avehicle to a destination and which is based on map information, andguidance means for providing guidance on said route by voice or withdisplay, said system comprising:travel-time-between-guidance-target-points determining means fordetermining either one of a travel time required for the vehicle totravel on a road between two successive guidance target points whoseinformation is extracted from the information stored in said guidancetarget point storing means and a speed limit on the road; continuousguidance threshold distance setting means for setting a thresholddistance for continuous guidance based on the travel time or the speedlimit determined by said travel-time-between-guidance-target-pointsdetermining means; distance-between-guidance-target-points calculatingmeans for calculating a distance between the two successive guidancetarget points whose information is extracted from the information storedin said guidance target point storing means; and route guidance meansfor providing continuous guidance through said guidance means when thecalculated distance between the two successive guidance target points isshorter than the threshold distance for continuous guidance set by saidcontinuous guidance threshold distance setting means.
 6. The vehiclenavigation system according to claim 5, wherein saidtravel-time-between-guidance-target-points determining means determineseither one of the travel time required for the vehicle to travel on theroad between the two successive guidance target points and the speedlimit on the road based on a travel time or a speed limit prestored ineach link information included in the map information, and saidcontinuous guidance threshold distance setting means sets the thresholddistance for continuous guidance based on a table showing a relationshipbetween the travel time or the speed limit and the threshold distancefor continuous guidance.
 7. The vehicle navigation system according toclaim 5, wherein said travel-time-between-guidance-target-pointsdetermining means determines either one of the travel time required forthe vehicle to travel on the road between the two successive guidancetarget points and the speed limit on the road based on a travel time orspeed limit prestored in each link information included in the mapinformation, and when said travel-time-between-guidance-target-pointsdetermining means determines that the determined travel time is equal toor less than a predetermined value or the determined speed limit isequal to or greater than a predetermined value, said continuous guidancethreshold distance setting means sets the threshold distance forcontinuous guidance to longer than a reference distance, and sets thethreshold distance for continuous guidance to shorter than the referencedistance otherwise.
 8. The vehicle navigation system according to claim5, wherein said travel-time-between-guidance-target-points determiningmeans determines either one of the travel time required for the vehicleto travel on the road between the two successive guidance target pointsand the speed limit on the road based on a travel time or speed limitprestored in each link information included in the map information, andsaid continuous guidance threshold distance setting means sets thethreshold distance for continuous guidance according to a relationalexpression showing a relationship between the determined travel time orthe determined speed limit and the threshold distance for continuousguidance.
 9. A route guidance method comprising the steps of: storinginformation about guidance target points placed on a route extendingfrom a current position of a vehicle to a destination; determining aconfiguration of a road between two successive guidance target points,whose information is extracted from said stored information; setting athreshold distance for continuous guidance based on the determined roadconfiguration; calculating a distance between the two successiveguidance target points; and providing continuous guidance when thecalculated distance between the two successive guidance target points isshorter than said set threshold distance for continuous guidance.
 10. Aroute guidance method comprising the steps of: storing information aboutguidance target points placed on a route extending from a currentposition of a vehicle to a destination; determining a travel timerequired for the vehicle to travel on a road between two successiveguidance target points whose information is extracted from said storedinformation; setting a threshold distance for continuous guidance basedon the determined travel time; calculating a distance between the twosuccessive guidance target points; and providing continuous guidancewhen the calculated distance between the two successive guidance targetpoints is shorter than said set threshold distance for continuousguidance.